The present invention relates to a method and apparatus for producing plasterboard with four tapered edges at the outer edge of each board.
The invention is in the field of continuous plasterboard production in which two sheets of cardboard-type paper, the lower one of which, because of its color is called the cream or facing paper (hereinafter the facing paper) and the other upper one being called the gray or backing paper are unrolled and come to enclose a plaster-based hydraulic binder inside a shaping means in the form of shaping plates or shaping rollers.
These two sheets of paper constitute, with the hydraulic binder, a composite strip which generally hardens on two hardening belts at the edges of which two narrow taper reservation strips are present allowing the future board to be provided with longitudinal tapered edges.
This composite strip is cut to the desired length, passed through the drying oven and is finally recut to its definitive length before being stacked. Such a method is well-known and will not be described further in detail in this specification.
The value of having tapered edges all around plasterboard has been known and appreciated for many years. Indeed, they facilitate the abutment and jointing of plasterboard and allow large surface area constructions to be built with excellent flatness characteristics and mechanical strength.
For many years now, it is known how to produce plasterboard with tapered longitudinal edges on continuous manufacturing lines but there is no method allowing production, on such continuous operating lines, of plasterboard with transverse tapered edges having the same characteristics as the longitudinal tapered edges.
There have obviously been numerous attempts and numerous patents have been taken out, essentially US patents, attempting to resolve this problem of continuous production of tapered transverse edges. U.S. Pat. No. 2,238,017 in particular attempted, in 1941, to create excess thickness by bending of cardboard but this invention never went into industrial development probably because the bending did not withstand the tension in the paper or the weight of the hydraulic binder. U.S. Pat. No. 2,991,825 attempted, 20 years later, to make imprints in a shaping roller system but this was not followed up by Industrial development certainly because the imprint generated got wiped out by crushing on the hardening belt.
It must be recognized that currently, plasterboard with tapered transverse edges if boards do exist are either the result of machining of the edges called tapering or feathering or artifices such as grooving, pressing and bonding of the edge, or, more frequently, are provided by the artisan who simply planes the edge of the board to provide a transverse taper.
The solutions are all expensive and impact on the production cost of the board with, additionally, the taper being far from having the qualities of the longitudinal taper. There is currently a real need on the market and a real call from the industry regarding this situation.